1. Field of the Invention
The present invention relates to semiconductor photodetectors, such as a PIN photodiode and an avalanche photodiode. More particularly, it relates to an improvement of such photodetectors made of a III-V alloy system, such as GaInAs/InP, and GaInAsP/InP.
2. Description of the Related Art
Photodetectors for optical fiber communication systems in the 1.3 to 1.6 .mu.m wavelength region include PIN photodiodes and avalanche photodiodes made of a III-V alloy system, e.g., GaInAs/InP system. Such a photodiode comprises an InP substrate, an InP buffer layer, an GaInAs light absorbing layer, and an InP cap layer, these layers being epitaxially formed in succession on the InP substrate. Heterojunctions are formed at interfaces between the InP buffer layer and GaInAs layer, and between the GaInAs layer and InP cap layer.
A PIN photodiode is produced, for example, in the following manner. First, an undoped (N.sup.- -type) InP buffer layer is epitaxially grown on a (100) oriented N.sup.+ -type InP substrate; an undoped (N.sup.- -type) GaInAs light absorbing layer is epitaxially grown on the InP buffer layer; and an N-type InP cap layer is also epitaxially grown on the GaInAs layer. A P-type region is formed as a light receiving area in the InP cap layer by a thermal diffusion process; an antireflecting film is formed on the P-type region; and a protecting film is formed on the InP cap layer. Finally, a P-side electrode and an N-side electrode are formed on the P.sup.+ -type region and the InP substrate, respectively. During the epitaxial growth, the growing conditions are controlled so that the lattice constant of the grown layers matches that of the InP substrate; to decrease the possibility of lattice mismatching. It is easy to match the lattice constant of InP layer to that of the InP substrate, since the InP layer and the InP substrate can have the same composition, but when the GaInAs layer is epitaxially grown on the InP layer, i.e., InP substrate, it, is necessary to control the composition ratio of Ga to In in the GaInAs layer so that the lattice constant of the GaInAs matches that of the InP. The thermal expansion coefficient of GaInAs is 1.24 times larger than that of InP; i.e., the thermal expansion coefficient of InP is 4.56.times.10.sup.-6 K.sup.-1 and that of GaInAs is 5.66.times.10.sup.-6 K.sup.-1. Therefore, when the GaInAs layer is grown at a temperature of, typically, from 600.degree. C. to 700.degree. C. to obtain a lattice match with the InP layer, the shrinkage of the GaInAs layer is more than that of the InP layer, at a room temperature, with the result that strain is generated, which causes mismatching between the layers. In another method, it is possible to grow the GaInAs layer in a mismatching condition so that the GaInAs and InP will match when cooled at room temperature.
In accordance with the above-mentioned two growing methods two GaInAs light absorbing layers are each epitaxially grown in liquid-phase condition on the InP buffer layer, and each of the obtained GaInAs layers is measured by an X-ray diffractometry, respectively. As a result, it was found that it is preferable to match GaInAs to InP at an epitaxial growing temperature, e.g., 600.degree. C. to 700.degree. C., as in the former method above, since this growing method generates less mismatching than the latter growing method above. Therefore, a Ga.sub.0.48 In.sub.0.52 As layer is liquid-phase grown on the InP layer at a usual growing temperature, e.g., 650.degree. C. under the lattice matching condition. In this case, at a room temperature, the lattice constance of GaInAs is smaller than that of InP, due to the thermal expansion difference, by a lattice mismatching degree .DELTA.a/a of approximately -0.07%: wherein ##EQU1## According to the above experience of the liquid-phase epitaxial growth it has been believed that, even in the vapor-phase epitaxial growth, the GaInAs layer growth under the lattice matching condition brings about the least mismatching at the heterointerface between the GaInAs and InP layers, notwithstanding the lattice mismatching degree (.DELTA.a/a) of -0.07% at a room temperature. Such a belief may be based on the fact that both the vapor-phase epitaxial growth and the liquid-phase epitaxial growth are epitaxial growths. However, although PIN photodiodes are produced at the same time by performing the vapor-phase epitaxial growth of the GaInAs layer under the above-mentioned preferable condition, the properties of the produced PIN photodiodes are not satisfactory, in particular, the problem of dark current in the photodiodes occurs frequently.